Peristaltic pumps have been used for a number of years for a wide variety of applications in a wide variety of environments. A peristaltic pump is typically designed to include a pump housing having a compressible pump tube disposed within the pump housing, where the pump tube generally forms a loop having an inlet end and an outlet end. An early version of the peristaltic pump used a straight, rather than round, track. The modern version using a pump tube forming a loop is a much more economically sound design with a smaller physical size and less costly to manufacture. The pump tube is typically filled with a fluid to be delivered by the pump from the inlet end to the outlet end. Fluid is caused to move through the pump tube by mechanical means, typically in the form of rollers, slides, cams, or cam-actuated fingers. In the case of rollers, they are typically driven by rotary means such as an electric motor or mechanically driven shaft. The rollers cause an occlusion of the pump tube by squeezing the pump tube against a wall or track within the pump housing, thereby forcing liquid or gas through the pump tube as the rollers move in a clockwise or counterclockwise direction.
One of the benefits of using a peristaltic pump is that the fluid does not come into contact with the operating environment, except within the pump tube, making the peristaltic pump ideal for medical applications, chemical testing, or other pump applications where it is important to eliminate contact of the fluid with the environment. Furthermore, the mechanical components of the pump do not come into contact with the fluid. As a result, the pump components remain free from contamination from the fluid being pumped. Thus, a peristaltic pump is easy to clean and sterilize because a pump tube may be simply discarded after use, and a new pump tube provided for the next use. In addition, it can be used at a variety of pump speeds, pump tube diameters, and can convey many types of fluids within the pump tube. However, one of the drawbacks associated with the peristaltic pump is that it has not been possible to provide a constant, or pulseless, flow of fluid through the pump tube. Pulses are caused when the rollers or occluding members exit occlusion, that is, when pressure of the roller is removed from the pump tube a vacuum or void is created in the pump tube. As the roller exits occlusion, the pump tube returns to its normal round shape and seeks to draw fluid from the outlet end of the pump tube to fill the void, resulting in a reduction in fluid velocity in the outlet line of the pump tube for the duration of the pulse. The potential for a negative pulse (reduction in fluid velocity) is created when any occluding member occludes the pump tube. This event displaces a specific fluid volume which is determined by the inner diameter of the pump tube, the track diameter, the shape of the occluding member, and to a lesser degree the wall thickness of the pump tube.
The lack of a constant fluid flow caused by pulses in the pump tube render the peristaltic pump unsuitable for certain precision applications. For example, in applications where a small volume of fluid is required, such as where less than a complete revolution of the rotor is used, the effect of the pulses are particularly undesirable. In addition, many sensors used in analytical instruments require a pulseless fluid stream so as to eliminate interference picked up by the sensors that could create an erroneous reading. Pulse dampeners on the pump inlet and/or outlet have been used in some applications. However, these devices work adequately for some applications, but they require a specific volume of liquid and they are very costly, particularly in large peristaltic pumps.
There have been several attempts to reduce the pulses caused when the rollers or occluding members exit occlusion of the pump tube. For example, in U.S. Pat. No. 3,358,609, the pump housing is designed to provide for the roller at the outlet end of the pump to exit occlusion gradually in an effort to minimize the pulsation caused by the rollers. In U.S. Pat. No. 5,470,211, the inlet end of the pump housing has an increasing radius of curvature in the direction of motion of the pump rollers and a continuously decreasing radius of curvature in the direction of motion of the pump at the outlet, where the radii of curvature are greater than the radius of curvature in the area between the inlet and outlet regions of the pump housing to provide a more gradual exiting of occlusion. However, in both the aforementioned patents, the roller directly upstream from the roller exiting occlusion maintains occlusion of the pump tube, continuing to occlude the pump tube, as the roller nearest the outlet end of the pump gradually exits occlusion, and otherwise fails to provide an additional compensating volume of fluid to fill the void. Thus, a vacuum or void continues to exist at the outlet end of the pump tube as the roller at the outlet end of the pump exits occlusion, without a quantity of compensating volume of fluid being provided to fill the void as the roller exits occlusion. In U.S. Pat. No. 3,726,613, a peristaltic pump is disclosed where a cam-controlled pusher is synchronized with the rollers at a location downstream from the rollers. The foregoing pump designs are not advantageous because they require costly manufacturing and/or the use of additional componentry such as a cam in addition to the rollers, or occluding members, and fail to provide a volume of compensating fluid to fill the void as the occluding member exits occlusion, or generally fail to eliminate the undesirable pulses caused when occluding members exit occlusion. Accordingly, there is a need to provide a peristaltic pump that is easy to manufacture, and operates to greatly reduce or eliminate the undesirable pulses caused when the rollers or occluding members exit occlusion.